This invention relates to a process for preparing of 3,4-dialkoxythiophenes and 3,4-alkylenedioxythiophenes by the decarboxylation of 3,4-dialkoxythiophenedicarboxylic acid and 3,4-alkylenedioxythiophenedicarboxylic acid, respectively, in a water-miscible polar solvent in the presence of copper catalyst under an oxygen atmosphere. As conductive polymers, polyanilines, polypyrroles and polythiophenes are used in various applications of electronic materials. In particular, polythiophenes have excellent physical properties.
Among them, poly(3,4-alkylenedioxythiophene) in which substituent(s) are introduced at the 3,4-position of the thiophene ring to improve solubility, thermal stability and chemical(solvent) resistance, has been developed by Bayer, Germany, and is used in various fields. Poly(3,4-alkylenedioxythiophene) can be used as                1) an antistatic agent;        2) an alternative to electrolytes in a condenser;        3) a coating on a printed circuit board; and        4) a hole-injecting layer in an organic electro-luminescence device.        
Poly(3,4-alkylenedioxythiophene) is generally prepared from the monomer 3,4-alkylenedioxythiophene. Thus, methods for synthesizing the monomer 3,4-alkylenedioxythiophene are also very important. The synthesis for a representative derivative, 3,4-ethylenedioxythiophene (EDOT), consists of four steps (step 1: condensation/step 2: substitution/step 3: hydrolysis/step 4: decarboxylation) starting from thiodiglycolate, as shown in Reaction Scheme (1):

Steps 1, 2 and 3 are general reactions that present no difficulties. On the other hand, step 4 (decarboxylation) should be improved because it has many shortcomings, including a low yield, complicated purification process, use of expensive solvent, and a high reaction temperature.
The method for the decarboxylation of 3,4-dialkoxythiophenedicarboxylic acid or 3,4-alkylenedioxythiophenedicarboxylic acid described by U.S. Pat. No. 2,453,103 (1948), E. Fager, [J. Amer. Chem. Soc., 1945, 67, 2217–8] and M. coffey et al., [Synth. Commun., 1996, 26, 2205–12] is generally carried out in the presence of copper catalyst (or Cu/Cr oxide) and quinoline solvent at high temperature (180˜200° C.) under a nitrogen atmosphere.
The general drawbacks of these prior arts can be summarized as follows:
1) The boiling point of quinoline is 238° C., while the boiling point of the final product (EDOT) is 225° C., and thus the product can not be isolated by fractional distillation. Instead, it must be isolated or purified by column chromatography, which is a drawback on an industrial scale.
2) The solvent quinoline should be avoided since it interferes with the workup process. Workup is carried out by washing with water and acid. The quinoline salt enters the waste-water, where it then causes environmental pollution or adds an additional step to recover the quinoline salt from the aqueous phase.
3) Since the reaction is carried out at high temperature (not lower than 180° C.) for a long period of time, substantial impurities such as tar-like materials are formed to give a bad yield (60% or less) or difficulties in purification.
According to E. Fager [J. Am. Chem. Soc., 1951, 2956–57], decarboxylation is performed by thermal decomposition at 190° C. in the presence of copper powder without solvent in vacuo (20˜40 mmHg), so that 3,4-dimethoxythiophene is prepared after distillation. This reaction is inappropriate for an industrial scale because it requires a considerable expenditure of energy and special equipment. A process for preparing 3,4-dimethoxythiophene from 3,4-dimethoxythiophenedicarboxylic acid by decarboxylation at high temperature (250° C.) without solvent or catalyst has also been reported [J. Prakt. Chem., 1996, 672–4], however, the yield is less than 65%.
U.S. Pat. No. 6,369,239 (2002) describes a process for preparing 3,4-dialkoxythiophenes or 3,4-alkylenedioxythiophenes by the decarboxylation of 3,4-dialkoxythiophenedicarboxylic acids or 3,4-alkylenedioxythiophenedicarboxylic acids, respectively, in the presence of high-boiling-point (230° C. or higher) solvent and copper salt catalyst (or no catalyst) under a nitrogen atmosphere. According to this process, decarboxylation is performed without catalyst by heating at 240° C. or higher for 24 hours, or in the presence of high-boiling-solvent and copper salt catalyst at 140° C. for 8 hours or more, and, similar to the prior arts it also gives a tar-like byproduct that impedes purification and reduces the yield. The solvent used in the other process is a water-immiscible solvent that has a higher boiling point than the final product (EDOT), which is usually isolated by fractional distillation under reduced pressure. Since the first purification by fractional vacuum distillation can give a mixture with a purity of about 50% (simultaneously distilled with the solvent), a second fractional vacuum distillation should be carefully carried out to obtain the final product in high purity. Thus, this process is complicated and gives low productivity.